Calcification of freshwater streams and the involvement of microorganisms in this process are still not fully understood. Here, we report on the metagenomic and metatranscriptomic analyses of bacterial community structures derived from an actively calcifying karst water creek biofilm and underlying tufa by employing next-generation sequencing technologies.The persistent and metabolically active bacterial communities were assessed by DNA-based and RNA-based 16S rRNA gene sequence analysis, respectively. We identified filamentous Cyanobacteria belonging to the Oscillatoriales as the predominant bacterial microorganisms in the biofilm. Cyanobacteria were accompanied by a high diversity of mainly aerobic members affiliated to different bacterial phyla. The second most abundant phylum was Proteobacteria, represented by the classes Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria. In addition, Chloroflexi, Planctomycetes, Actinobacteria, Acidobacteria, Bacteroidetes, Spiroch...